1. Field of the Invention
This invention relates to testing of electrical distribution circuits, and particularly to a tester that will verify the existence of wiring problems such as an insulation failure or grounded/shared neutrals.
2. Related Art
The common type of circuit breaker used for residential, commercial and light industrial applications has an electromechanical thermal magnetic trip device to provide an instantaneous trip in response to a short circuit and a delayed trip in response to persistent overcurrent conditions. Some circuit breakers of this type include ground fault protection, which trips the circuit breaker in response to a line-to-ground fault and, in some cases, a neutral-to-ground fault. Ground fault protection is provided by an electronic circuit which is set to trip at about 4 to 6 milliamps of ground fault current for people protection, and at about 30 milliamps for equipment protection. It is known to incorporate a test circuit in the circuit breaker, which tests at least portions of the electronic ground fault test circuit. It is also known to test for proper wiring connections. Test circuits for this purpose are commercially available. One such circuit is described in U.S. Pat. No. 6,072,317, assigned to the assignee of this application.
More recently, interest has arisen in providing protection against arc faults. Arc faults are intermittent, high impedance faults which can be caused, for instance, by worn insulation, loose connections, broken conductors and the like. Arc faults can occur in the permanent wiring, at a receptacle or, more likely, in the wiring of loads or extension cords plugged into the receptacle. Because of their intermittent and high impedance nature, they do not generate currents of sufficient instantaneous magnitude or sufficient average current to trigger the thermal-magnet trip device which provides the short circuit and overcurrent protection within a circuit breaker.
Arc fault detectors are generally of two types. One type responds to the random high frequency content of the current waveform generated by an arc. The other basic type of arc fault detector responds to the step increase in current occurring as the arc is repetitively and randomly struck. Examples of arc fault detectors of the latter type are disclosed in U.S. Pat. Nos. 5,224,006 and 5,691,869. Built in test circuits have also been proposed for such arc fault detectors. U.S. Pat. No. 5,459,630 discloses several forms of built in test circuits for such arc fault detectors. In one embodiment, in which the arc fault detector utilizes a coil to sense current, the test circuit adds a capacitor which forms, with the impedance of the coil, an oscillator generating wave form with an amplitude which simulates the rapid rise of a step change in current produced by an arc. In another embodiment, the user must repetitively close a switch, which connects a resistor between the line conductor and neutral to again generate large amplitude pulses.
While arc fault and ground fault circuit breakers will trip on ground or arcing fault conditions, they do not necessarily indicate where the fault is in a real installation. One difficulty is that the circuit breaker containing the detectors is located at a load center together with the circuit breakers for other circuits in the installation. However, the fault condition can occur anywhere downstream. Also, there may be some loads that cause nuisance tripping when a fault current does not exist.
There is a need, therefore, for improved test circuits for electrical distribution systems that can verify the integrity of branch wiring.
There is also a need for verifying the proper operation of an arc fault or ground fault circuit that has responded to a fault current condition, especially faults which are remote from the detectors, to assure the problem is within the circuit and not within the detector. Additionally, there is a further need for such a testing circuit that will assist in identifying the location of the fault. Furthermore, there is need for such testers which are flexible, simple and economical.
These needs and others are satisfied by this invention, which is directed to a tester that has two modes of operation, an insulation failure test mode and a shared/grounded neutral test mode. In the insulation failure test mode, a relatively large AC voltage source supplying a relatively small current, in the order of approximately 3 to 5 milliamps, is selectively applied to a pair of wires in the circuit under test. The magnitude of the voltage source that is applied is substantially greater than the line voltage normally applied to the circuit, but less than the voltage rating of the wiring insulation. A current meter monitors the leakage current flowing in the test circuit. If the leakage current is greater than approximately 3 milliamps, then an insulation failure exists.
In the shared/grounded neutral test mode, a pulsed low voltage source is applied across the ground and neutral conductors of the circuit under test. A portable ammeter then monitors any current flowing through either the neutral or ground conductors, starting at a location relatively near the voltage source, to detect the flow of a pulse current. If a pulse current is detected, the portable ammeter is moved along the conductor in a direction away from the source. The point at which the pulse current vanishes will identify the location of the fault. Both tests are conducted with the main power to the circuit and any load disconnected.